A conventional receiving apparatus is explained. One example of reception data determination methods in digital communications is a maximum-likelihood-determination method. According to this method, a reception apparatus calculates a metric of replica generated from a transmission path response and a transmission symbol candidate and a reception signal, searches a replica that minimizes the metric from among all combinations, and outputs a corresponding transmission symbol candidate as a determination result. This maximum-likelihood-determination method has excellent reception performance. However, since a metric is calculated for all available combinations of replicas, an enormous amount of operation is necessary.
For example, Nonpatent Literature 1 describes about “Sphere Decoding (hereinafter, SD)” as a technique for decreasing the amount of operation in the maximum-likelihood-determination method. According to this technique, hypersphere is set around a reception signal point in a lattice point space formed by all replicas, and a metric is calculated for only replicas that are present inside the hypersphere. According to this technique, replicas that are present at the outside of the hypersphere do not need to be considered. Therefore, the number of times of metric calculations can be decreased from that required by the maximum-likelihood-determination method. An initial value of a hypersphere radius is given in advance based on, for example, dispersion of noise. The radius of the hypersphere is updated by a minimum metric each time when metric is calculated. The determination process ends when the radius becomes small and when no replica is present within the hypersphere along with progress of the process. A candidate of a transmission signal having a minimum metric is output as a determination value. When the SD is adopted, characteristics equivalent to those obtained by the maximum-likelihood-determination method can be obtained with a small amount of operation.
Nonpatent Literature 1: Emanuele Viterbo, Joseph Boutros, “A Universal Lattice Code Decoder for Fading Channels,” IEEE Transactions on Information Theory, Vol. 45, No. 5, pp. 1639-1642, July 1999.